The invention relates to a microwave feeder or applicator, i.e., a conductive chamber for coupling to an electromagnetic microwave generator, arranged to receive a sample of material which is consequently processed by a microwave electromagnetic field.
It can be shown, from Maxwell's equations, that any cavity filled with a homogeneous dielectric has stationary or resonating stages ("natural modes") for a discrete set of frequencies of microwaves called natural resonance frequencies. It is thus possible, in the case of substantially cylindrical cavities, to calculate the electric field and the magnetic field in the cavity at various natural anodes.
The difficulty is that since the microwave feeder contains a sample, its behaviour differs from that of a cavity filled with a homogeneous dielectric--i.e., the feeder contains a first dielectric space (that occupied by the sample) and a second dielectric space, i.e., the air surrounding the sample. The dielectric characteristics of the sample are invariably quite different from those of air, and the introduction of the sample modifies the resonance state which can be calculated when the feeder is filled with air only.
In practice, if the volume of the sample is very small compared with that of the cavity, e.g., in the case when the dielectric constants of materials are measured the small modification produced by introducing the sample is calculated by a method of successive approximations, which gives satisfactory results.
However, the size of the sample is inevitably important in the case of microwave treatment, as opposed to measurement. Consequently the method of successive approximations is unsuitable for microwave feeders.
In the simple, conventional example of microwave kitchen ovens, where the dimensions of the food to be cooked may vary and the volume of the oven is large, the number of possible natural resonance modes is quite high. In preference to optimising the energy output of the various modes in dependence on the various sizes of sample, an average is obtained of all the possible modes, using a mode mixer. Usually the mixer comprises a fan having metal blades which rotate and thus deform the volume offered to the microwaves. The resulting microwave feeder offers a large volume but is not resonant in operation, and thus cannot subject the processed product to a powerful electric field.
It is also known to construct resonating microwave feeders in the form of a cylindrical cavity having a circular cross-section, for treating a cylindrical sample disposed coaxially to the cavity, or a parallelepipedal cavity through which the cylindrical sample extends from one side to the other. However, these feeders are inevitably small if they are to be resonant in operation, and therefore cannot be used to treat large samples in a powerful microwave electric field.
The invention has an object to provide a substantially cylindrical resonating microwave feeder suitable for elongate transversely-disposed samples.